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Posted: December 21, 2007

Pulse shaping could hold the key to secure quantum networks

(Nanowerk News) Communication based on the quantum state of light and/or matter has received a lot of attention in recent years. The promise is that successful communication also guarantees secure communication—which is something that makes people lift their heads from the desk, massage their foreheads, and listen to us boring physicists. Unfortunately, information sent in this way is also encoded rather delicately—often on single photons—and communications channels always loose some photons, meaning that lots of closely spaced repeaters are necessary. The heart of the problem is that the whole system is critically dependent on a bunch of mostly unknown parameters, which means that every link has to be fine-tuned. However, a new way of looking at the system has led to an insight which may eliminate the critical dependence on some of those parameters.

Two researchers from Nanjing University looked at the behavior of a particular type of link where information is first encoded on a quantum dot, a small lump of material that acts like a single atom ("Quantum communication in a network with serious imperfections" – open access article). This quantum dot sits between two mirrors, so when it radiates its information, the photon is "caught" by the mirrors and deposited into a fiber optic cable. The fiber optic cable transmits the photon to a second quantum dot that also happens to be sitting between two mirrors. In this case, the mirrors "catch" the photon and bounce it off the quantum dot until it finally absorbs it. At this point, information transfer has occurred.

The key to the system is something called the Rabi frequency. This frequency is determined by the interaction between the quantum dot and the mirrors that it sits between. Basically, an excited quantum dot will, after a certain amount of time and with a certain probability, emit a photon that will be reflected back and forth between the mirrors. Later, the quantum dot will, with a certain probability, reabsorb the photon and the process will repeat.

If you perform this experiment repeatedly, you will find a characteristic rate at which the photon cycles between being in the cavity or existing as an electron excitation in the quantum dot. This is important because the duration of a light pulse needs to be the correct multiple of that characteristic time, otherwise the photon will not be absorbed. Unfortunately, everything that happens to the pulse as it traverses a fiber optic system changes the pulse duration, making it much harder to transmit information between the two quantum dots.

The innovation that the researchers present is the idea of modifying the shape of the pulses that control the quantum dots. This will then counteract some of the errors induced by the fiber network between the two dots. The team's calculations indicate that some pulse shapes result in very robust entanglement between the two quantum dots, providing a secure, long distance link.

Unfortunately, I can't help thinking that different sources of distortion require different pulse shapes, and it would be a very rare network that had only one type of distortion. Furthermore, the researchers need to put down their pencils and get into the lab to put this to the test. In the end, I think pulse shaping will play a role in making communication links based on quantum entanglement more robust, but it will play a small part in solving a hugely complicated problem.